Generally, CMOS active pixel sensors utilize amplifiers built in each pixel to amplify photovoltaic signals generated by photosensors in response to illumination of light, which can be readout selectively according to an X-Y address of each pixel. For such CMOS active pixel sensors, the photovoltaic signals are amplified by the built-in amplifiers before being transmitted to external control circuits, thereby eliminating noises associated with transmitting passes of the signals.
It is known that the sensitivity of an active pixel sensor is determined by at least three factors. The first factor is related to the area in the active pixel sensor available for converting photons to electrons. An increase in the area leads to an increase in the amount of charges generated. A second factor is related to the capacitance for the integration of the charges sensed by the active pixel sensor. Theoretically, a voltage on a capacitor for a given amount of charges is inversely proportional to the capacitance of the capacitor. Accordingly, when the capacitance increases, the voltage decreases for the same amount of charges. A third factor is the charge-to-voltage gain of the readout amplifier. In a display with built-in pixel sensors, a source follower is typically used as a charge-to-voltage amplifier. However, the gain is substantially equal to one, in practice the gain is less than one due to characteristic of the transistors.
FIG. 10 illustrates a conventional CMOS active pixel sensor 10 that has three N-channel MOS transistors 1, 4 and 5. In this active pixel sensor 10, a photodiode 2 employed as a photosensor has a anode connected to the ground, and a cathode connected to both the source of the reset transistor 1 and the gate of the readout transistor 4. An integral capacitor 3 is connected between the anode and cathode of the photodiode 2. The gate of the reset transistor 1 is connected to a reset line. Both the drain of the reset transistor 1 and the drain of the readout transistor 4 are connected to a supply voltage, VDD. The source of the readout transistor 4 is connected to the drain of the row select transistor 5. The gate and source of the transistor 5 are connected to a row select line and a column output line, respectively. One end of the column output line is connected to a terminal of a current source 6, whose other terminal is connected to the ground.
A timing diagram corresponding to the operation of active pixel sensor 10 is depicted in FIG. 11. The active pixel sensor 10 is first reset by a RESET signal, during a reset stage, which turns on the reset transistor 1 to place the supply voltage VDD on the cathode of the photodiode 2. An integration stage begins when the RESET signal makes a transition from HIGH to LOW where photo-generated electrons are collected on the photodiode 2 to reduce the voltage on the cathode of the photodiode 2 from the value VDD placed there during the reset stage. When a ROW SELECT signal transits from LOW to HIGH, the active pixel sensor 10 starts a readout status. During the readout status, the ROW SELECT signal is asserted to turn on the select transistor 5 to place the voltage at the source of the readout transistor 4 on the column output line for detection. The voltage on the gate of the readout transistor 4 formed by the charge accumulated on the cathode of the photodiode 2 will be followed by the source of the readout transistor 4.
For such an active pixel sensor 10, its sensitivity can be improved by increasing the size of the photodiode 2 and/or the readout transistor 4. However, simply increasing the size of the photodiode 2 and/or the readout transistor 4 will reduce the aperture ratio and the light transmittance of cells of a display. For a fixed amount of brightness of the display, the brightness of the backlight illuminating the display must be increased, thereby increasing the consumption of power of the display.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.